Historically, snowmobiles having multiple rear tracks have been limited in manufacture and use. As such, suspension systems for such arrangements have also been limited in manufacture and use. An example of a known suspension system for a snowmobile having multiple rear tracks is taught by Uchida U.S. Pat. No. 5,014,805. Historically, snowmobile suspension assemblies such as the Uchida '805 suspension assembly were typically designed for traversing flat terrain and as such were not designed for or used much in mountain or hill climb environments. The Uchida '805 suspension assembly resides within dual rear tracks and includes a suspended frame structure to which rollers are rotatably mounted. The assembly is mounted to the frame by various arms and linkages that define the suspension travel and allow the tracks to move up and down relative to the vehicle. The dual rear tracks are located on each side of the snowmobile and are spaced apart outside of the elongated saddle seat, on the outboard sides of the vehicle. The rear tracks are separated by a power train, including an engine and a drive train. In addition, a muffler system and an air cleaner and carburetor are located between the tracks. Disc brakes are applied to axles disposed transversely through the dual rear tracks.
Over the years, the basic design of snowmobiles has significantly changed from that disclosed in Uchida '805. Snowmobiles have more commonly been designed using a single driven inboard closed-loop rear track typically located in a 17-inch width or narrower tunnel beneath the rear portion of the snowmobile chassis. Contrary to the Uchida '805 snowmobile, the engine and drive train for snowmobiles are now typically positioned towards the front portion of the vehicle. The track typically rides along a plurality of rails and idler wheels and is driven by a single drive shaft located near the front portion of the track.
Upon this single-track arrangement, many developments and improvements related to rear suspension systems have occurred. Each of these improvements has been particularly geared towards the use of a single driven, suspended track and each typically includes various suspension components such as shocks, springs, coupling devices and front and/or rear linkages. For example, Yasui et al U.S. Pat. No. 4,407,386 teaches a rear suspension system which comprises both front and rear suspension assemblies connected to a pair of spaced apart guide rails. The guide rails carry spaced rollers and backup a single drive track. A series of linkages, shocks and springs move relative to each other to resiliently support and dampen the movement of the drive track and soften the ride of the snowmobile.
Boivin et al U.S. Pat. No. 5,860,486 relates to a rear suspension system for a snowmobile that has a pair of primary pivoting arms that interconnect a series of rails with the undercarriage of the snowmobile chassis. Primary suspension means and secondary suspension means are mounted to a first horizontal bar and a second horizontal bar respectively to support a single track and the weight of the vehicle. The various linkages, suspension means and pivoting arms cooperate to create a softer ride and accommodate weight transfer on the snowmobile.
Savage et al U.S. Pat. No. 6,263,991 relates to a rear suspension for a snowmobile which includes two elongated members of substantially constant length coupled to the snowmobile frame and to a single endless track guide rail assembly. The suspension keeps the guide rail assembly at a substantially constant angular orientation relative to the frame for improved performance and operator comfort.
Boivin et al U.S. Pat. No. 6,234,264 also relates to a snowmobile suspension system which includes front and rear suspension arms connected to side rails and a biasing assembly, all of which are configured to suspend a single drive track on a snowmobile while providing a smoother ride.
However, the single-track arrangement of the prior art has many disadvantages. For example, when operating a snowmobile in mountain terrain, the snowmobile is commonly required to traverse the side of a steep hill. To maintain a snowmobile having a single suspended track at a level position, the operator positions both feet on one running board such that the entire weight of the snowmobile bears on one edge of the single track. This requires a great effort on behalf of the operator. It also results in minimized floatation and performance, which is a result of large pressures on one edge of the track and a minimal contact area that minimizes traction and the ability to apply power to the ground.
Another disadvantage with single track suspensions is found in hill climb racing, where an operator drives the snowmobile up a steep hill that is covered with moguls. As the snowmobile traverses the hill, the snowmobile is propelled off of moguls and typically lands in an uneven vehicle roll position. In conventional single track snowmobile suspensions this activity results in high moments into the vehicle that tend to pitch the rider.
It is therefore highly desirable to provide an independent suspension system for a snowmobile that is capable of supporting the snowmobile while independently articulating multiple rear tracks through a defined suspension travel. It is desirable to provide an independent suspension system that is simple, easy to construct and requires minimal parts, thus minimizing mechanical breakdown. It is further desirable to provide an independent suspension system that is adjustable and that maximizes performance while minimizing overall system weight. It is also desirable to provide such an independent suspension system that is sized to occupy and operate within a 17-inch or narrower tunnel beneath the snowmobile seat.